Crustaceans – Discover-Nature

Seaside

Shorelines of oceans, seas, and large bodies of water around the world are habitats for plants and animals that thrive in challenging conditions. Tides occur twice each day when water levels rise over a six hour period to cover large areas before receding to leave those same areas open to the air until the next tide starts to move inland again.

^{Nearshore habitat zones by King County, WA;} ^{Littoral Zones by US Government}

Shoreline habitat has been classified into zones often known by many different names. The intertidal zone is the area affected by changing temperature and salinity conditions as water moves in and out with each tide. The subtidal zone is the lowest and is exposed to air only during extreme spring tides or storms. The backshore zone is the highest and is exposed to water only during extreme spring tides or storms.

^{Starfish & anemone in tidal pool by Jonathan Levy}

^{Sea urchin in tidal pool, Bahia de Banderas by Jim Hoffman, Mar 2017}

Many species of animals live in each zone and are adapted to living completely submerged as well as completely uncovered by water for several hours each day. They handle the change in temperature and salinity with exposure to air or water. Tidal pools may keep some animals submerged for far longer periods. Temperature, dissolved oxygen, and salinity change far less for these organisms.

Bryozoans by John Turnbull, Sep 2021; Sea anemone by Barb Ignatius; Squat lobster, Glathea rostrata, by FWC Fish & Wildlife Research Institute, Aug 2013

Some species can be found in every zone in the ecosystem, but many are anchored to rocks or dug into the bottom. Species may prefer one zone, but get washed into another zone by wave action. Many organisms have tough outer surfaces to put up with battering waves and exposure to sun and wind. All of the occupants are subject to a wide range of predators from those that move on dry land to underwater hunters. Let’s take a look at a few of the creatures able to live in these ever-changing conditions.

The sea anemone clings to rocks and protects itself by drawing in its tentacles to become a jelly-like blob. The squat lobster can be found under stones along the shoreline. Its tail is fan-shaped and facilitates quick escapes when the animal is threatened by pulling the lobster through the water with powerful strokes. Bryozoans are small filter feeding organisms protected by a hard exoskeleton that the animal can withdraw into for protection.

^{Dog whelk, Nassarius livescens, by Budak, Dec 2016}

A mollusk called a dog whelk is a stealth predator often found on rocky surfaces. It uses its tongue to drill through the shells of other mollusks, where it squirts a digestive juice into the prey’s shell which kills and partially digests the prey. The dog whelk then uses its tongue to suck up the soupy meal. The starfish is another fierce predator. It wraps its arms around a shellfish and pulls it apart. Upon opening the shell, the starfish pushes its stomach out of itself and into the prey’s open shell where it empties its digestive juices inside, and similar to the dog whelk, creates a tasty, soupy, meal.

Sea urchins can be found clinging to hard surfaces where they feed on algae and other small, encrusted animals. Fan worms look like a leathery tube among underwater rocky crags. Several feathery tentacles fan out into the water to filter out microorganisms flowing by.

Shorelines create a habitat that is constantly changing with tides moving in and out, inconsistent weather conditions, and a large variety of flora and fauna coming and going. Learn more about this fascinating habitat at the Jurica-Suchy Nature Museum or a natural history museum near you.

^{Botanical beach at Port Renfrew by James Wheeler; Feather Boa kelp in the surf by Kqedquest, May 2007; Rocky intertidal zone in Oregon by Minustide, May 2015}

The Ocean Deep

Seventy percent of the Earth’s surface is under the ocean, and only a small portion is in shallow water along continental shelf areas. The ocean is divided into five zones; the sunlight zone, above 650 feet; the twilight zone, extending to 3300 feet; the midnight zone, down to 13,000 feet; the abyssal zone, down to 21,300 feet; and the hadal zone, extending to the bottom of the ocean. Below 3,000 feet, no light penetrates from the surface; the water temperatures are freezing; and oxygen levels are very low. The pressure exerted on any surface at sea level is 14.7 pounds per square inch (psi), and humans can withstand three to four times this amount, or 45 psi to 60 psi. At 3,000 feet deep, the pressure is 1,345 psi.

^{Delta submersible by Robert Schwemmer, CINMS, NOAA, Oct 2010}

Pressure keeps sea water from becoming solid ice, even when it is at freezing temperatures. It also makes exploring the ocean difficult, requiring special submersible vehicles. Everything moves more slowly under pressure, and a submersible can take an explorer deep into the ocean given enough time and oxygen. Deep sea creatures can survive because the structure of their bodies is lean and contain no air spaces. Let’s take a look at a few of the wondrous creatures that live in these habitats.

^{Humpback anglerfish by August Brauer in Public Domain}

In the darkness of deep areas, many species find food and mates by touch. Others may use bioluminescent communication – the ability to create light from a chemical reaction between luciferin and oxygen within an organism’s body. Lights show up as spots of various colors on different parts of the body and may be used for attracting a mate, confusing a predator, or attracting prey. Lanternfish have several spots located on their heads, undersides, and tails. Female anglerfish and dragonfish have a shaft that protrudes from their head out in front of their mouths with a glowing tip attracting prey within striking distance. Gulper eel use the lighted tip of their tail as a lure to attract a meal.

^{Tripod fish by NOAA Okeanos Explorer Program, Public domain, 2010}

^{Fangtooth fish by Sandra Raredon-Smithsonian Institution, Public domain}

Tripod fish have two pelvic fins and a tail fin that are elongated and can be filled with fluid to stiffen them, allowing the fish to stand on the ocean bottom while conserving energy. Two long, wavy pectoral fins act as arms to detect prey floating or swimming by and push the prey toward their mouths. Fangtooth fish is another species that can detect vibration and nearby movement. This species is only six inches in length, but have the largest teeth in proportion to their body size of any fish in the ocean. The long lower fangs fit into pouches in the roof of their mouths so their teeth do not pierce their brain cavity.

^{Dragonfish teeth array by UC San Diego Jacobs School of Engineering, May 2019}

The dragonfish, another small fish about six inches long, has a frightening array of teeth located on its jaws and tongue. Their teeth are transparent and their bodies are black, making them impossible to see even in depths where there may be some light. The jaws are loosely hinged, allowing the fish to open its mouth wider than normally hinged jaws would allow. Prey swimming nearby may not notice the wide opening, nor be able to detect the transparent teeth, making the dragonfish a top predator of invertebrates, squid, and shrimp. In addition, when biting, they inject a highly toxic poison to paralyze and kill their prey. This toxin can be dangerous to humans getting bitten or attempting to eat this species.

^{Giant spider crab by Michael Coghlan, Jan 2014; Gulper eel by Claf Hong, Mar 2005; Lanternfish, Myctophum punctatum1 by Emma Kissling}

Some fish practice diel vertical migration, a pattern of movement where a species feeds near the surface at night and moves back into deeper waters to rest during the day. Lanternfish come up to feed on zooplankton and fangtooth fish feed on squid in shallow waters. Some species, including anglerfish and gulper eels, have elastic-like skin for their mouths and stomachs. Mouths can be opened extremely wide to swallow prey larger then themselves. The food is contained in a similarly elastic stomach which shrinks slowly as digestion progresses. Several different species of spider crabs roam the ocean depths scavenging for meat or plant material that falls to the bottom.

Deep-sea animals are seldom seen, but are not immune from climate change and human impacts. Scientists continue to study changes in deep-sea food webs caused by overfishing, ocean acidification, and expanding low-oxygen zones. Learn more about the habitat and species of these areas at the Jurica-Suchy Nature Museum at Benedictine University or your local nature museum or aquarium.

Moon Jellies

Moon jellies are a group of ten species in the genus Aurelia. They are considered true jellies, but DNA testing is required to distinguish between the species. They are found in oceans worldwide in shallower waters common in coastal and upwelling areas where prey is more abundant.

Moon jellies have a shallow, transparent, dome-shaped body and short tentacles that hang down from underneath. Full size adults grow from 12″ to 16″ in diameter. As with all jellyfish, their body is 95% water, and many of the organs found in other animals are missing. Jellyfish lack respiratory, circulatory and excretory systems. They have no lungs, heart, blood, brain, bones, nor teeth. They do have rhopalia, small sensory organs found around the rim of the dome, containing statocyst structures that sense gravity to determine balance and depth in the water.

Moon jellies by Eric Kilby at New England Aquarium, 2009

Dietary requirements of moon jellies are not well understood. Jellyfish are dependent on what the wind and currents bring by to eat. Stinging cells call cnidocytes cover their tentacles and contain nematocysts that can deliver toxin to immobilize prey. Moon jellies are predators hunting fish, mollusks, crustaceans, and other small invertebrates. Food is collected in small pockets along the rim of the dome. There are four arms that transfer the food to the gastrovascular cavity located in the middle of the underside. Digestive enzymes break down the food and nutrients are distributed throughout the body by a system of canals.

Moon jelly, *Aurelia aurita* by James St. John at Lake Superior Aquarium, 2015

Reproduction includes both sexual and asexual processes. Adult males release strands of sperm into the water which adult females will ingest and use to fertilize their eggs. She incubates the eggs in pockets located on her four arms. Eggs hatch and larvae, called planulae, are released into the water. Planulae continue to grow and eventually attach themselves to rocks on the ocean floor. Anchored to the rock, they enter their next life cycle stage known as a polyp. At this time, they can catch, eat and digest their own prey, and may spend several years as a polyp. As a polyp, they begin strobilation, an asexual reproduction process whereby a polyp clones itself and buds off the clone to form an ephyra. Both the polyp and ephyra grow into adult jellyfish. Through an unknown process, polyps can determine when conditions are ideal to enter their final adult form, called a medusa, through sexual reproduction. Strobilation allows the population to expand quickly, while egg production guarantees a diversity of genes in the next generation.

Moon jelly, *Aurelia aurita* by Keppet at Monterey Bay Aquarium, 2013

Moon jellies are social animals, travelling in smacks, large groups numbering in the thousands that float across great distances. An adult in the wild will live about a year, but may live several years in captivity. During summer, jellyfish try to reproduce daily. At the end of summer, with less food available and energy levels waning, many individuals become susceptible to bacterial diseases and die before winter. In addition, they are food for open ocean predators including ocean sunfish and sea birds. Leatherback sea turtles feed exclusively on moon jellies, and can eat several hundred at each meal.

Moon jelly, *Aurelia aurita* by Roger Myers at Monterey Bay Aquarium, 2003

In areas of increased human activity, moon jellies tend to thrive due to a decrease in competitors for food. Moon jellies do well in waters with low dissolved oxygen concentrations and high nutrient loads. When food is limited, moon jellies can shrink to one-tenth of their full size, returning to their previous size as food opportunities become more abundant. A recent study identified the moon jelly as one of the “immortal jellyfish” groups. Several individuals have been observed reversing the aging process, becoming younger all the way back to sexual immaturity, and then aging back to adults. Studies hypothesize that this is an unending cycle. You can view these incredible creatures at many of the world’s aquariums including the Shedd Aquarium in Chicago.